The present invention relates to a method for purifying acrylic acid. Particularly, it relates to a method for purifying acrylic acid from an aqueous solution of acrylic acid, while preventing polymerization of acrylic acid. More particularly, it relates to a method whereby an operation for purifying acrylic acid can be carried out constantly over a long period of time while preventing polymerization of acrylic acid in a distillation column at the time of removing low boiling point components such as water and acetic acid by means of a dehydration column from a crude aqueous solution of acrylic acid obtainable by catalytic oxidation of e.g. propylene.
As a typical method for producing acrylic acid, a method may be mentioned which comprises oxidizing propylene and/or acrolein by a molecular oxygen-containing gas in the presence of steam by means of an oxidizing catalyst. A crude aqueous solution of acrylic acid can be obtained by cooling and/or absorbing by water, reaction gas thus obtained. This crude aqueous solution of acrylic acid contains, in addition to acrylic acid, by-products such as acetic acid, formic acid, formaldehyde and acetaldehyde. Among these by-products, one most abundantly formed and being accordingly particularly important at the time of purification, is acetic acid. It is believed to be not so efficient to separate water, acetic acid and acrylic acid directly by distillation in view of their chemical similarity and the physicochemical properties such as gas-liquid equilibrium.
Accordingly, it has been common to employ, as a method for purifying acrylic acid, a method which comprises subjecting water to dehydration distillation by means of an organic solvent azeotropically distillable with water (which may sometimes be hereinafter referred to as xe2x80x9can azeotropic solventxe2x80x9d) and further separating acetic acid by distillation.
For the distillation process to carry out such separation of water and acetic acid, a method wherein both will be separated simultaneously by a single distillation column (hereinafter referred to as xe2x80x9ca single column methodxe2x80x9d) and a method wherein they are separated by means of the respective distillation columns (hereinafter referred to as xe2x80x9ca double column methodxe2x80x9d), are conceivable, and many proposals have been made for the respective methods, as follows.
(1) With respect to the single column method JP-B-46-18967, JP-B-46-29372, JP-B-46-22456, JP-B-46-34692, JP-B-49-21124, JP-A-5-246941, etc.
(2) With respect to the double column method
JP-B-41-15569, JP-B-46-18966, JP-B-50-25451, JP-B-63-10691, JP-A-3-181440, JP-B-6-15495, JP-B-6-15496, JP-A-8-40974, etc.
These two methods have the following merits and demerits.
The single column method is intended to separate water and acetic acid simultaneously by a single distillation column, whereby it is required to employ a distillation column having a large plate number, and a large reflux ratio will be required. Accordingly, this method is disadvantageous from the viewpoint of energy. Further, as the plate number increases, the column bottom pressure tends to be high, and the column bottom temperature will accordingly be high, but it is not desirable to subject readily polymerizable acrylic acid to such a high temperature.
Whereas, in the double column method, water and acetic acid are separated by means of distillation columns respectively, whereby the optimum distillation conditions and distillation column can be employed, and this method is advantageous also from the viewpoint of energy. Further, this method has a merit such that acetic acid as the main by-product can be recovered from the distillation column for separation of acetic acid. Further, the plate number of each column can be reduced, and the column bottom temperature can be made low, hereby this method is preferred also with a view to prevention of polymerization of acrylic acid.
However, also in the process for purifying acrylic acid by this double column method, polymerization of acrylic acid has been likely to take place especially in the vicinity of the bottom of the dehydration column, whereby a stabilized operation has been still difficult.
To overcome such difficulty, JP-A-8-40974 proposes a method of controlling the concentrations of water and an azeotropic solvent in the bottom of the azeotropic dehydration column. However, this method has been still inadequate to carry out a continuous operation for a few months under a stabilized condition.
It is an object of the present invention to provide operational conditions to prevent polymerization of acrylic acid and make it possible to operate the distillation column under a stabilized condition for a long period of time, when an aqueous solution of acrylic acid is subjected to dehydration by means of a dehydration column.
The present inventors have made various studies on distillation conditions by paying a particular attention to the fact that the above-mentioned polymer of acrylic acid will deposit at a specific site, whereby a continuous operation of the dehydration column for a long period of time will be impossible. As a result, they have found it possible to present polymerization of acrylic acid in the distillation column by controlling the temperature of the specific site in the dehydration column within a specific range, and thus have arrived at the present invention.
That is, the present invention provides a method for purifying acrylic acid, which comprises carrying out dehydration of an aqueous solution of acrylic acid by means of a dehydration column, wherein a distillation column having a theoretical plate number of at least 3 plates is used as the dehydration column, and the operational temperature of a site corresponding to the second theoretical plate is adjusted to be from 50 to 78xc2x0 C.
The present invention also provides the above-mentioned method for purifying acrylic acid, wherein the operational temperature of the site corresponding to the second theoretical plate is adjusted to be from 60 to 73xc2x0 C., and the above-mentioned method for purifying acrylic acid, wherein the bottom temperature of the dehydration column is adjusted to be from 60 to 90xc2x0 C.
Further, the present invention provides the above-mentioned method for purifying acrylic acid, wherein the aqueous solution of acrylic acid is a crude aqueous solution obtained from a reaction gas formed by catalytically oxidizing propylene and/or acrolein by molecular oxygen, and the above-mentioned method for purifying acrylic acid, wherein the acrylic acid concentration of the aqueous solution of acrylic acid is at least 40 wt %.
Furthermore, the present invention provides the above-mentioned method for purifying acrylic acid, wherein an organic solvent azeotropically distillable with water is used at the time of the dehydration distillation.